1. Field of the Invention
The present invention relates to a projection exposure apparatus for causing the sensitive layer coated on a substrate, such as a water for fabricating semi-conductor device and a glass plate for fabricating liquid crystal display device, to be exposed with the patterned image on a mask or reticle. More particularly, the invention relates to a projection exposure apparatus provided with an off-axis type alignment system whereby to observe mark patterns and others on a substrate through an objective optical system dedicatedly fixed outside a projection optical system or a projection optical system only.
2. Related Background Art
An example of the conventional projection exposure apparatus provided with an off-axis type alignment system (hereinafter expediently referred to as stepper) is disclosed in U.S. Pat. No. 4,452,526 or in Patent Abstracts of Japan, Vol. 2, No. 92, Jul. 28, 1978, p. 436 E78, JP-A-53-56975.
Also, the fundamental concept of the off-axis system is disclosed in a paper entitled "ALIGNING SEMICONDUCTOR MASKS" in IBM Technical Disclosure Bulletin Vol. 13, No. 7, December 1970, p. 1816. Further, in U.S. Pat. No. 4,677,301 filed by the same assignee hereof for the present invention, there is proposed an alignment method for a sensitive substrate using both of two alignment systems of the off-axis type. One of the alignment systems is a system for detecting the mark on the sensitive plate appearing at a predetermined position in the image field of the projection optical system only through the projection optical system which performs a focusing of the original pattern on the mask or (reticle) onto the sensitive plate (hereinafter referred to as inner-field off-axis alignment system). The other is a system for detecting the mark on the sensitive plate through the dedicated objective optical system fixed to the outside of the projection optical system (hereinafter referred to as outer-field off-axis alignment system). The provision of such two kinds of off-axis alignment systems is also disclosed in the aforesaid U.S. Pat. No. 4,452,526.
The above-mentioned conventional apparatus has a fiducial plate with a fiducial mark as a reference fixedly on the wafer stage which is secondarily movably by a step and repeat method. This fiducial plate is used for measuring the distance between the off-axis alignment system and projection optical system, that is, the base line value between them. Now, in conjunction with FIG. 1, the principle of the conventional base line measurement will be described. In this description, the off-axis alignment system is assumed to be of the outer-field type.
In FIG. 1, a main condenser lens ICL illuminates the reticle (mask) R evenly at the time of exposure. The reticle R is supported by a reticle stage RST. This reticle stage RST is driven to enable the center CC of the reticle R to be aligned with the optical axis AX of the projection optical system (hereinafter referred to as projection lens) PL. Meanwhile, on a wafer stage WST, the fiducial mark FM, which is equivalent to the alignment mark formed on the wafer surface, is provided, and when the stage WST is positioned to allow this fiducial mark FM to arrive at a predetermined position in the projection field of the projection lens PL, the mark R on the reticle R and fiducial mark FM are detected at the same time by the alignment system DDA of a TTL (through the lens) type provided above the reticle R. The distance La between the mark RM and the center CC of the reticle R is a predetermined value defined at the time of designing. Therefore, the distance between the projection point of the mark RM on the image formation side (wafer side) of the projection lens PL and the projection point of the center CC becomes La/M, where M is a magnification of the projection lens PL observed from the wafer side to the reticle side, and in the case of a 1/5 reduction projecting lens, M=5.
Also, on the outside (outer field of projection) of the projection lens PL, an off-axis wafer alignment system OWA is fixedly mounted. The optical axis of the wafer alignment system OWA is in parallel with the optical axis AX of the projection lens PL on the side of the projection image plane (on wafer plane). Then, inside the wafer alignment system OWA, there is provided a target mark TM on the glass plate, which serves to be the reference when the mark on the wafer or the fiducial mark FM is aligned. This target mark TM is arranged substantially in conjugation with the projected image plane (wafer surface or the surface of the fiducial mark FM).
Now, the base line value BL is obtained as shown in FIG. 1 by measuring the position X.sub.1 of the stage WST when the reticle mark RM and fiducial mark FM are aligned as well as the position X.sub.2 of the stage WST when the target mark TM and fiducial mark FM are aligned by the use of an interferometer and others and then calculated the difference (X.sub.1 -X.sub.2) This base line value BL will be a reference value when the mark on the wafer is transferred to the position just below the projection lens PL by an alignment conducted by the wafer alignment system OWA later. In other words, given the distance between the center of one shot (an area to be exposed) on the wafer and the mark on the wafer as XP, and the position of the wafer stage WST when the wafer mark and the target mark TM are matched as X.sub.3, the wafer stage WST should be driven to the position obtained by an expression given below in order to coincide with the center of the shot with the center CC of the reticle. EQU X.sub.3 -BL-XP (or X.sub.3 -BL+XP)
In this respect, this expression represents the position in only one dimensional direction as a principle, and in practice, a two-dimensional consideration should be given. Moreover, the calculating method should be different depending on the arrangements of the TTL alignment system DDA (that is, the arrangement of the mark RM), wafer alignment system OWA, and others.
In any case, subsequent to having detected the mark position on the wafer using the wafer alignment system OWA of the off-axis type, the wafer stage WST is driven for a predetermined amount. Hence conducting exposure immediately after the pattern on the reticle R has been overlapped with the shot area on the wafer accurately.
In a conventional technique such as described above, when the positional relationship between the detected center point (the center of the target mark TM) of the off-axis alignment system OWA and the projection point of the mark RM on the reticle R by the projection lens PL (base line value BL) is measured, its relative distance is obtained by a laser interferometer while driving the wafer stage WST. Consequently, due to the running accuracy of the wafer stage WST, air fluctuation on the optical path of the laser beam of the laser interferometer, and other unavoidable causes, there is a limit to the improvement of the precision with which to measure the base line value. Also, it is necessary to drive the wafer stage WST for positioning the fiducial mark FM in the detection area in the TTL alignment system DDA and also to drive the wafer stage WST for positioning the fiducial mark FM in the detection center of the off-axis alignment system OWA. Thus there is a limit to the increase in the speed with which to execute the base line measurement process.
Further in the conventional stepper, the extended line of the measuring axis (beam optical axis) of the laser interferometer for measuring the position of the wafer stage WST is simply set to be perpendicular to the optical axis of the projection lens both in the direction X and direction Y as shown in U.S. Pat. No. 4,677,301. Accordingly, it is considered difficult to implement the direction of the mark detection so that Abbe's error (sine error) is always zero when the off-axis alignment system OWA is used for detecting various marks. Then, it may also be considered to provide a combination of laser interferometers by arranging a set of a laser interferometer thereby to make the Abbe's error zero with respect to the optical axis of the projection lens and a set of a laser interferometer thereby to make the Abbe's error zero with respect to the detection center of the off-axis alignment system OWA.
The application for a patent on an apparatus for which a conception such as this is materialized has been already filed by the inventor et al hereof and is issued as U.S. Pat. No. 5,003,342.
In this case, the two sets of the laser interferometers are used by switching them for the stage position measurement for the wafer alignment using the off-axis alignment system OWA and for the stage position measurement at the time of projection exposure. However, the adjustability (coordination) of the values in measuring both positions must be taken into account. Otherwise, errors may result inevitably.
In the above-mentioned U.S. Pat. No. 5,003,342, in order to effectuate the adjustability required, one set of the interferometer is reset while the fiducial mark plate is positioned just below the projection lens and the other set of the interferometer is reset while the fiducial mark plate is positioned just below the off-axis alignment system. Nevertheless, during each operation of the two sets of the interferometers, the wafer state is caused to be driven for the predetermined amount. Strictly speaking, there still remain errors in the running accuracy of the wafer stage, particularly depending on its yawing, and errors due to the air fluctuation (refraction index disturbance) in the optical path of the laser interferometers.